There are a number of known methods for accurately positioning a member on a surface of an object. For example, FIGS. 1 and 2 illustrate a known method for positioning a member 212 on an object 214 having a planar surface 216. As shown in FIGS. 1 and 2, two spaced-apart, round pins 218 are attached to the member 212 such that they extend therefrom to be received in corresponding round holes 220 provided in the object 214. To ensure that the pins 218 can be inserted into the holes 220, the distance "X" between the centers of the pins 218 and the distance "Y" between the centers of the holes 220 must be precisely controlled so that those distances are substantially equal. Such precision machining is costly and time consuming. As such, to accommodate slight variations between those distances, which can result in misalignments between the pins 218 and the holes 220, the holes 220 are typically fabricated with larger diameters than the outside diameters of the pins 218. However, that action can lead to positioning inaccuracies depending upon the magnitude of the clearance provided between the pins 218 and their corresponding holes 220. Also, to fasten the member 212 to the object 214 separate threaded bore(s) (not shown) are typically provided in the object 214 that are adapted to receive corresponding capscrew(s) (not shown) that extend through bores (not shown) provided in the member 212. Such fastening arrangement requires the clearance between the threaded capscrew and the bore in member 212 to be larger than the tolerances between the pins 218 and the bores 220 to ensure that the positioning of the member 212 relative to the object is solely controlled by the pin and bore arrangement.
Another known positioning method for positioning an object 312 on a member 314 having a planar surface 316 is illustrated in FIGS. 3 and 4. When using that method, two round holes 320 are provided in the object 314. A round pin 318 and a "diamond" shaped pin 322 are attached to the object 312 such that they extend therefrom. Such diamond shaped pins 322 typically have their lower end portions configured to a so-called diamond shape by machining the cylindrical surface of the pin on opposite sides, leaving only small arcs spaced on opposite sides on the pin at the original diameter of the pin. Slight variations between the distance "X'" between the centers of pin 318 and diamond shaped pin 322 and the distance "Y'" between the centers of the round holes 320 along a reference line "Z--Z" connecting the center points of the bores 320 is largely accommodated by the tapered sides 324 of the diamond shaped pin 322 as long as the line "W--W" connecting the mid-points of the arcuate faces of the diamond pin 322 are substantially perpendicular to line "Z--Z".
Over the years, the above-mentioned positioning methods have been used to locate a fixture or other workholding device on a support member such as a tooling plate or grid plate during high precision machining operations. For example, U.S. Pat. No. 4,585,217 to Erickson teaches the use of differently shaped pins for aligning a tooling block adapted to support one or more workpieces thereon on a planar base plate that is attached to a rotary table of a computer numerically controlled machining device. Two of the pins that are used each have a dome-shaped upper portion and a cylindrical lower stem that is adapted to be received in a corresponding bore in the base plate. The tooling block has at least two alignment bores that are adapted to receive the upper dome-shaped portions of those pins therein to facilitate initial alignment of the block on the base plate. Another pin, having a lower cylindrical stem portion adapted to be received in a corresponding bore in the base plate and an upper portion having a frusto-conical shape adapted to be received in a corresponding bore provided in the block is also used to facilitate alignment of the block on the plate. In addition, another pin that also has a cylindrical lower stem portion adapted to be received in corresponding bore in the plate and a diamond-shaped upper portion adapted to be received in a corresponding bore in the block is used. That collection of pins is used to precisely locate the tooling block on the base plate.
A somewhat similar arrangement of pins is taught in U.S. Pat. No. 4,834,358 to Okolfschan et al. In that device, adapter members are attached to a grid plate and are sized to support a workpiece thereon at a desired height. Locating pins, each having a locating portion and a cylindrical stem portion that is adapted to be received in a corresponding bushing provided in the adapter are provided to locate the workpiece in a predetermined position. The locating portion of each pin is adapted to be received in corresponding bores provided in the workpiece. Each pin may be further provided with a shoulder that cooperates with a corresponding screw arrangement to prevent the pin from rotating in its corresponding bushing. One or more of the pins may be provided with a "diamond" cross-sectional shape.
Both of the locating pin arrangements disclosed in the patents mentioned immediately above have inherent inaccuracy problems. In particular, the locating pins each have cylindrical "stem" portions that, in one case, serve to locate the pins in a base member and, in the other case, serve to locate the pins in an adaptor member that is attached to a base plate. Thus, the degree of positioning accuracy of one member (i.e., a tooling block or a workpiece) relative to the other member (i.e., a base plate or an adaptor attached to a base plate) is directly dependent upon the accuracy of each of the cylindrical pin stems and the corresponding bore in which it is to be received.
Every machining process has a certain amount of inaccuracy associated with it due to, perhaps, limitations of the apparatus used to machine the bore, wear of that apparatus' components or the particular machining method employed. As such, each bore will have a range of diameters (i.e., a tolerance) that could result from that particular machining apparatus or process, each of which would be deemed acceptable, in view of the inherent inaccuracies associated with that apparatus and/or process. Similar inaccuracies are also encountered when machining the cylindrical lower stems of the locating pins. Thus, the accuracy of the pin relative to the first member in which its lower cylindrical stem portion is received will depend upon the accuracy of the pin and the accuracy of the bore into which it is inserted. For example, a pin member having a diameter of 0.500" and a tolerance of .+-.0.002" that is adapted to be inserted into a bore of 0.504" having a tolerance of .+-.0.002" could cause the members to be misaligned by as much as 0.004" if the pin diameter is 0.498" and the bore diameter is 0.506".
Likewise, U.S. Pat. No. 4,968,012 to Haddad et al. discloses a modular workpiece holding apparatus for holding a workpiece in a predetermined position. The apparatus includes a base plate that has a plurality of bores therein that each have a lower threaded portion. The apparatus also includes a plurality of support members that each have a mounting base with four bores formed therein. A bushing is secured in each bore in the mounting base such that a portion of each bushing protrudes from its bottom surface. Two bushings that each have a circular exterior portion that protrude from the bottom of the mounting base and two bushings that each have a diamond-shaped exterior portion that protrudes from the mounting base are used to position the mounting base relative to the base plate by inserting the protruding bushings into bores provided in the base plate. The support member is secured to the base plate by threaded fasteners that extend through the bushings to threadedly engage the threaded portion of the bores in the base plate. This arrangement of locating bushings, however, also has the same inaccuracy problems of the pin arrangements that were discussed above. In particular, because the bushings must be inserted into holes provided in the mounting base of each support member, the positioning accuracy of the mounting plate relative to the base plate is dependant upon the accuracy of the outer surfaces of the bushings and the accuracy of the holes in which they are to be inserted.
Also, in those arrangements wherein precisely shaped locating protrusions formed on an object are adapted to be inserted into corresponding bores or hollow bushings in another object, it is typically difficult to separate the objects due to the precision fit created therebetween. Thus, depending upon the size of the individual objects, they usually must be manually manipulated such that the locating protrusions do not bind against the interior surface of the bores in which they are received while the objects are urged apart. Such method of separation is awkward and time consuming.
In view of the foregoing, there is a need for apparatus that can more accurately position an object on the planar surface of a member.
There is a further need for apparatus that can more accurately position stationary clamping elements onto the base member of a precision workholding device.
There is a further need for apparatus that has the above-mentioned attributes and that can minimize the amount of effort and manipulation required to detach the stationary clamping elements from the device.